1. Field of the Invention
The present invention relates to a ring network system and a network node.
2. Description of the Related Art
Ethernet (registered trademark) in layer 2 defined by IEEE 802.3 is available as an aspect of a network such as a switching hub that learns MAC address information in a reception frame and is configured by connecting a network apparatus controlling a transmission destination of a frame.
As a protocol for giving redundancy to a network configuration based on Ethernet, ANSI/IEEE Std 802.1D, 1998 Edition: 8. The Spanning Tree Algorithm and Protocol describes STP (Spanning Tree Protocol). In the STP, a control frame called BPDU (Bridge Protocol Data Unit) is exchanged with a regular interval, and a state called blocking is set in a port on a network, so that an occurrence of a loop in Ethernet is prevented. A path switching is realized by cancelling the blocking upon detecting a change in a state of the network (topology) on the basis of a link failure occurrence and the like.
Japanese Patent No. 4958746 recites that “each node on a network is provided with a transmission quality monitor unit 35 monitoring a transmission quality of the node with a regular interval, a transmission quality management table 42 storing the monitored transmission quality, a termination determination unit 39 determining a termination portion on the basis of information in the transmission quality management table 42, and a node state management unit 40 managing a node state of the node. Each node transmits the transmission quality information about the node monitored by the transmission quality monitor unit 35 to all the nodes, so that the transmission qualities about all the nodes on the network are stored in the transmission quality management table 42 to be shared, and on the basis of a comparison result of the transmission qualities, a node having the worst transmission quality is determined to be a termination portion” (see Abstract).